1. Field of the Invention
The present invention relates to a power supply, and more particularly, to a power supply for relieving spikes.
2. Description of the Prior Art
Conventional passive elements have their respective operating voltage ranges or operating current ranges. When the conventional passive elements are not operated under their operating voltage ranges or their operating current ranges, the passive elements do not operate normally or exceed their loadings so that permanent damage is caused. For neutralizing these defects resulting from applying inappropriate operating voltages or inappropriate operating currents, certain controlling mechanisms are used on various circuits for limiting their operating voltages or operating currents within appropriate ranges. Various techniques are also utilized for protecting passive elements inside a same circuit.
Please refer to FIG. 1, which is a diagram of a conventional power supply 100, where a feedback controlling mechanism is utilized on the power supply 100 with the aid of constant voltages and constant currents. The power supply 100 includes a transformer 102, a diode 104, a first capacitor 106, an inductor 108, a second capacitor 110, a first resistor 112, a switch 114, a plurality of light emitting diodes 116 connected in series, a second resistor 118, a third resistor 120, a feedback circuit 122, and a pulse width modulation (PWM) integrated circuit 124. An input voltage source Vin is coupled to a first input terminal of the transformer 102. The diode 104 has a first terminal coupled to a first output terminal of the transformer 102. The first capacitor 106 has a first terminal coupled to a second terminal of the diode 104, and a second terminal coupled to both ground and a second output terminal of the transformer 102. The inductor 108 has a first terminal coupled to a second terminal of the diode 104. The second capacitor 110 has a first terminal coupled to a second terminal of the inductor 108, and a second terminal coupled to the second terminal of the first capacitor 106. The plurality of light emitting diodes 116 has a first terminal coupled to a second terminal of the inductor 108. The first resistor 112 has a first terminal coupled to the second terminal of the second capacitor 110. The switch 114 may be an N-type metal oxide semiconductor transistor or a P-type metal oxide semiconductor transistor. The switch 114 has a first terminal coupled to a second terminal of the plurality of light emitting diodes 116, a base coupled to a pulse width modulation signal source PWM2, and a second terminal coupled to a second terminal of the first resistor 112. The second resistor 118 has a first terminal coupled to the second terminal of the inductor 108. The third resistor 120 has a first terminal coupled to a second terminal of the second resistor 118, and a second terminal coupled to ground. The feedback circuit 122 includes a first operational amplifier 126 and a second operational amplifier 128. The first operational amplifier 126 has a positive input terminal coupled to a reference voltage source Vref, and a negative input terminal coupled to the second terminal of the switch 114. The first operational amplifier 126 is utilized for receiving a voltage difference across the first resistor 112, where the voltage difference is generated from a constant current outputted from the second terminal of the switch 114 and passing through the first resistor 112. A constant current controlling mechanism of the power supply 100 is accomplished by comparing the reference voltage source Vref with the voltage difference across the first resistor 112 with the aid of the first operational amplifier 126, where the constant current is generated from a feedback voltage, which is generated by the voltage at the first output terminal of the transformer 102 and the plurality of light emitting diodes 116. The second operational amplifier 128 has a positive input terminal coupled to the reference voltage source Vref, and a negative input terminal coupled to the first terminal of the third resistor 120. The second operational amplifier 128 is utilized for receiving a divided voltage at the intersection of the second resistor 118 and the third resistor 120, where variable resistance is inducted from the second resistor 118 and the third resistor 120. The divided voltage is generated as part of the voltage at the first output terminal of the transistor 102, and is outputted into the feedback circuit 122. When the voltage at the first output terminal of the transformer 102 is constant, the divided voltage must be a constant voltage also. Therefore, the constant voltage controlling mechanism of the power supply 100 is implemented by comparing the reference voltage source Vref with the constant divided voltage with the aid of the second operational amplifier 128. The PWM integrated circuit 124 has an input terminal coupled to an output terminal of the feedback circuit 122 so as to generate a pulse width modulation signal PWM1 according to a control signal, which is generated by the feedback circuit 122 according to the constant voltage controlling mechanism or the constant current controlling mechanism. The PWM integrated circuit 124 has an output terminal coupled to a second input terminal of the transformer 102 so as to control both a duty cycle and an output voltage of the transformer 102 according to the generated pulse width modulation signal PWM1. Primary characteristics of the power supply 100 shown in FIG. 1 lie in the constant voltage controlling mechanism and the constant current controlling mechanism, each of which takes AC or DC input voltages and cooperates with two operational amplifiers respectively.
When the pulse width modulation signal PWM2 is high, the switch 114 is switched on so that the plurality of light emitting diodes 116 connected in series is conducted. At this time, with the aid of the constant voltage and current controlling mechanisms, the duty cycle of the pulse width modulation signal PWM1 is lengthened so that the duty cycle of the output signal of the transformer 102 is lengthened as well, and the loading of the plurality of light emitting diodes 116 is thus increased. Therefore, when the pulse width modulation signal PWM2 is low then, the switch 114 is switched off so that the plurality of the light emitting diodes 116 is not conducted. At this time, according to the constant voltage and current controlling mechanisms, the duty cycle of the pulse width modulation signal PWM1 is shortened so that the duty cycle of the output signal of the transformer 102 is shortened as well, and the loading of the plurality of light emitting diodes 160 is decreased. With alternative high and low of the pulse width modulation signal PWM2, average loading of the plurality of light emitting diodes 116 is decreased to a certain degree.
However, under such controlling mechanisms, instant and severe variations in voltage or current may still damage the plurality of light emitting diodes 116. Please refer to FIG. 2, which is a schematic diagram of the operating voltage, i.e., the voltage difference, across the plurality of light emitting diodes 116 versus the voltage level of the pulse width modulation signal PWM2 while the plurality of light-emitting diodes 116 is under external luminance-modulation. The term “external luminance-modulation” indicates the function of modulating luminance of light-emitting electronic products, such as televisions, with the aid of external devices, such as remote controllers. For example, the operating voltage of a single light emitting diode 116 is assumed to be 3.5 volts, and the number of the plurality of light emitting diodes 116 is assumed to be 10. Therefore, the operating voltage of the plurality of light emitting diodes 116, which varies with the voltage level of the pulse width modulation signal PWM2, ranges between 30 and 40 volts. However, when the pulse width modulation signal PWM2 is switched from low to high, i.e., from off to on, violent voltage variation is generated on the operating voltage of the plurality of light emitting diodes 116, where the voltage variation is illustrated in the form of area surrounded with a dotted line in FIG. 2. The voltage variation inducts additional burst current into the plurality of light emitting diodes 116 so that the lifespan of the plurality of light emitting diodes 116 is shortened.
Please refer to FIG. 3, which is a schematic diagram of the operating voltage of the plurality of light emitting diodes 116 versus the operating current of the plurality of light emitting diodes 116 while the plurality of light-emitting diodes 116 is under external luminance-modulation. The curve LED indicates the characteristic curve of a single light emitting diode 116, whereas both the other curves LED1 and LED2 define a supposed upper bound and a supposed lower bound of the characteristic curve of the plurality of light emitting diodes 116. Since different light emitting diodes 116 have slightly different characteristics because of fabrication procedure, the characteristic curve of the plurality of light emitting diodes 116 has difference with the characteristic curve of a single light emitting diode 116 and lies between the characteristic curves LED1 and LED2. In other words, when the plurality of light emitting diodes 116 is biased according to the characteristic curve of a single light emitting diode 116 without taking differences in fabrication procedure into consideration, unexpected burst voltages and burst currents are generated so that the plurality of light emitting diodes 116 suffer damage and shortened lifespan of different degrees.